The production of block hydrocarbon copolymers incorporating at least one block of predominately polymerized vinylaromatic hydrocarbon compound, e.g., styrene, and at least one block of predominately polymerized conjugated alkadiene, e.g., butadiene or isoprene, is well-known in the art. The block copolymer products are also well-known and a number are commercial, being marketed by Shell Chemical Company as KRATON.RTM. Thermoplastic Rubber. Although the block copolymers are prepared in a variety of geometric types, e.g. linear, branched, radial or star, many of the more useful block copolymers are linear.
In one modification, the linear polymers are produced by a sequential polymerization. A three-block or triblock polymer is illustratively produced by polymerizing a block of vinylaromatic hydrocarbon (conventionally termed an A block) in the presence of a polymerization initiator, typically an alkyllithium. A conjugated alkadiene is subsequently provided to the polymerization mixture without deactivation of the reactive vinylaromatic hydrocarbon polymer chain. Upon completion of the alkadiene polymerization to form a poly alkadiene block (conventionally termed a B block), vinylaromatic hydrocarbon is again provided. Polymerization of the vinylaromatic hydrocarbon produces a third block. The lithium-terminated active or "living" polymer chain is then deactivated and the polymer is recovered by known methods. This type of polymer is conventionally known as an ABA polymer or, if styrene and butadiene are the monomers, as an SBS polymer.
It is also well-known and conventional to produce linear block copolymers by the coupling of the living polymer chains. In such instances, smaller living chains are produced by sequential polymerization and then coupled through the use of a coupling agent. For example, a block of polymerized vinylaromatic hydrocarbon in prepared in the presence of an initiator and conjugated alkadiene is added to produce a polyalkadiene block. The living polymer chains so produced are then coupled. The production of linear block copolymers is often achieved by coupling procedures when polymer of high molecular weight is desired or when it is desired that the end blocks have the same chemical composition and substantially the same molecular weight. To prepare these types of linear block copolymers, the use of sequential polymerization is technically or economically unattractive.
A number of coupling agents to be use in the production of linear block copolymers are well-known, including silicon or tin compounds, aromatic diisocyanates, ester, dialdehydes and diketones. However, certain of these coupling agents, particularly the silicon or tin agents, contain halogen which results in the presence of alkali halides, particularly lithium halide, in the coupled product. Such halides are corrosive and frequently lead to corrosion problems in the processing of the polymer product. Other coupling agents, e.g., the diisocyanates, are toxic. Ester-coupled products frequently lack thermal stability and dialdehydes and diketones have relatively low coupling efficiency.
It is also known to employ a polyfunctional coupling agent but to adjust the relative quantities of coupling agent and living polymer chain to encourage the production of linear coupled polymer product. However, such polyfunctional agents lead to a mixture of coupled products including linear, branched and non-coupled products. For example, the use of trialkylphosphites as disclosed by Kahle et al., U.S. Pat. No. 3,803,266, leads to products other than linear block copolymer. It would be of advantage to provide an improved coupling process for the production of linear block copolymers of vinylaromatic hydrocarbon and conjugated alkadiene wherein the use of coupling agent provides a relatively high coupling efficiency and selectivity to linear block copolymers.